1. Field of the Invention
This invention relates to ferromagnetic iron oxides, more specifically, to a process for producing a ferromagnetic material having good stability to pressure and heat, a high coercive force which undergoes little change with time, and a narrow coercive force distribution, which is capable of providing magnetic tapes with improved SP (print through) ratios and erasure characteristics.
2. Description of the Prior Art
This invention relates to an improvement in the methods shown, for example, in Japanese Patent Publications 5,515/61, 4,825/62 and 49,475/74 wherein ferrite is precipitated on acicular particles, and in the methods shown, for example, in Japanese Patent Applications (Laid Open) 108,599/74, 37,667/75 and 37,668/75, wherein increased coercive force is attained by precipitation of a Co-containing compound in solution. Such iron oxides having a high coercive force obtained merely by a precipitation in an aqueous solution have better stability to heat and other properties with the passage of time, such as the print through ratio, than a magnetic material obtained by heating Co-containing iron oxide at temperatures as high as 300.degree. to 500.degree. C to diffuse Co uniformly in the particles. Such a method involves simple process steps, and is suitable for commercial produciton.
However, there is a saturation point in the coercive force, and even by increasing the amount of Co added, it is extremely difficult to raise the coercive force to more than 500 to 600 Oe. Thus, the resulting iron oxide is not entirely satisfactory for use in magnetic tapes designed for high density recording.